1. Field of the Invention
The present invention relates generally to an electronic control unit, and more particularly to the protection of a module housed in an electronic control unit.
2. Description of Related Art
A circuit module used as an electronic control unit generally comprises circuit chips having different functions, such as an integrated circuit (IC), a high density integrated circuit (LSI), and a microcomputer and the like, attached by die bonding or the like to a circuit board upon which an electric circuit pattern has been printed, and is connected to the wiling on the circuit board by wire bonding or soldering. As circuit chips, both a packaged chip which is covered by a package made of plastic or the like and sealed with a junction coating resin, and a bare chip having no such sealing are used. In a packaged chip, an electrode pad and a lead of the package are connected by wire bonding while the lead and the wiring of the circuit board are connected by soldering. In a bare chip, however, the electrode pad and the wiling of the circuit board are directly connected by wire bonding.
When the above circuit module is used under conditions in which it is exposed to high temperature or large amounts of vibration or shock, such as in an electronic control unit housed in an engine room of a vehicle, it is necessary to take some precaution to ensure the protection of the circuit module against corrosion, shock, and impact. In one such measure for protection of circuit modules, in which both the bare chip and packaged chip types are used together to form an electric circuit, the bare chip on the circuit board is surrounded by a frame into which a gelatinous silicone or epoxy resin or the like is then filled to a certain height so as to cover the bare chip. An example of this type of circuit module is disclosed in Japanese Patent Application Laid-open Publication (xe2x80x9cKokaixe2x80x9d) No. 10-303232.
Protecting only the bare chip portion in this way, however, is not always sufficient for use under extreme conditions such as those mentioned above. One conceivable measure for improving shock and impact resistance is to surround not only the bare chip portion but the entire circuit module with a viscoelastic potting gel that absorbs vibration and impact, in which case, the entire electronic control unit case which houses the circuit module would be filled with the potting gel. However, if the entire housing (ECU case) is filled with potting gel, because the wires connected to the bare chip portion are thin, there is the possibility that they may become disconnected by the increase in load on the connections due to viscoelastic vibration of the potting gel. Moreover, because filling with the potting gel is the last step after incorporating the circuit module into the case and wire bonding it to a connector portion mounted to the case, the bare chip is not protected while the circuit module is handled until the last step.
It is therefore a first object of the present invention to provide an electronic control unit in which a bare chip of the circuit module is sealed by a simple method.
It is another object of the present invention to fill an entire case, housing a circuit module, with potting gel and to provide an electronic control unit wherein there is no oscillation of wiring of the bare chip by the gelatinous substance even if there is a large impact or vibration.
In order to achieve the foregoing objects, in one aspect the present invention provides an electronic control unit comprising an electronic circuit module, in which a bare chip is mounted on a circuit board, and a case for housing the module. The case is filled with a potting gel to protect the module. In advance of the filling with potting gel, the bare chip is sealed with a gelatinous substance which is capable of retaining its own shape, which has thixotropy and which covers wire connections between the bare chip and the circuit board. The gelatinous substance is cured (hardened) and then sealed within the potting gel.
Because a gelatinous substance having a thixotropy is used to seal the bare chip, the bare chip, which has a lower resistance than other portions, is locally protected by a simple step involving only application of a gelatinous substance. Moreover, the fact that the seal assumes a shape on its own due to the thixotropy of the gelatinous substance obviates the need for arranging a frame or the like to shape the seal, as is necessary in the related art. Further, because the seal around the bare chip portion does not require extra space, the circuit board is able to be made smaller, thus enabling costs to be reduced. In addition, by first sealing the bare chip portion in a clean room, it is possible to perform the transfer and assembly steps thereafter in an ordinary environment. This is advantageous for reducing costs due to simplification of the manufacturing line.
In another aspect the present invention provides an electronic control unit comprising an electronic circuit module, in which a bare chip is mounted on a circuit board, and a case for housing the module, in which the case is filled with a potting gel to protect to the module, as in the first embodiment. However, in the second embodiment the bare chip and wire connections between the bare chip and the circuit board, are sealed inside the potting gel, with the cured gelatinous substance which is a hardened inner layer Gxe2x80x2 having a penetration less than that of the potting gel G. Therefore, even if the potting gel which fills the case vibrates viscoelastically, the vibration is blocked by the gelatinous substance, which has a low penetration, and does not reach the bare chip. As a result, the bare chip, as well as the connections of thin wires, are reliably protected without need for special shielding. Further, because the bare chip is sequentially sealed by first the gelatinous substance and then the potting gel which forms a layer on the outside of the gelatinous substance, heat radiates to the case through these seal layers, enabling the bare chip to be cooled.
In all embodiments, the sealing layer remains gelatinous (unhardened or xe2x80x9csemi-solidxe2x80x9d) in the final product, i.e., after the potting gel has been hardened.
It is preferred that the gelatinous substance be a junction coating resin which, upon curing, has a penetration that becomes less than that of the potting gel with heat-hardening.
Accordingly, a seal is formed around the bare chip with only the smallest possible amount of material due to the thixotropy of the gelatinous substance. This enables the bare chip portion to be reliably sealed using a junction coating resin which has a high cost and which is more reliable than potting gel, while keeping costs down.